Potable water reservoirs such as standpipes (normally tanks with height greater than diameter), ground storage tanks (normally tanks with height less than diameter) or elevated tanks are connected to water distribution systems and are used, among other things, to supply water to the systems and/or maintain the pressure in the systems during periods when water consumption from the system is higher than the supply mechanism to the system can provide. The reservoirs are therefore usually filling during periods when the system has supply capacity that exceeds the current consumption demand on the system or discharging into the system when the system has supply capacity that is less than the current consumption demand on the system. Potable water reservoirs typically contain water which has been treated through the addition of a disinfectant to prevent microbial growth in the water. Disinfectant concentrations in stored water decrease over time at a rate dependant upon a number of factors. This can result in unacceptable water quality if the period of retention of the water, or any part thereof in the reservoir, becomes too long or if the incoming fresh, treated water is not properly mixed with the existing stored water. Therefore, the age or retention period of water within potable water reservoirs and the mixing of incoming fresh water with the existing water are of concern to ensure that the quality of the water will meet the regulatory requirements for disinfectant concentrations. In addition, during periods of below freezing weather, the top surface of the water will cool and may freeze (this is referred to as an ice cap) unless it is exchanged for or mixed with the warmer water entering the reservoir. An ice cap may become thick enough to adhere to the reservoir walls and span the entire surface even when the water is drained from below. If sufficient water is drained from below a fully spanning ice cap, a vacuum is created, collapsing the ice cap which in turn can create, during the collapse, a second vacuum which can be much larger than the reservoir venting capacity and can result in an implosion of the roof and possibly the upper walls of the reservoir.
Water reservoirs are often filled and drained from a single pipe or a plurality of pipes located at or near the bottom of the reservoir. Under these conditions, when fresh water is added to the reservoir, it enters the lower part of the reservoir and when there is demand for water in the system, it is removed from the lower part of the reservoir resulting in a tendency for the last water added to be the first to be removed. This can be referred to as short circuiting. Temperature differences between stored water and new water may cause stratification which can in turn exacerbate short circuiting and water aging problems. Filling and draining from a single or a plurality of pipes located at or near the bottom creates little turbulence particularly in areas within the reservoir remote from these inlet and outlet pipes. As a result, the age or residency time of some waters within parts of the reservoir can be very long, resulting in loss of disinfectant residual, increase in disinfection by-products, biological growth, nitrification and other water quality and/or regulatory issues. This is referred to herein as “stagnation” or “stagnant water”. A perfect system would provide a first in, last out scenario (“cycling”), however, perfect cycling is either not possible or is cost prohibitive. A preferred system provides a tendency toward cycling combined with a first mixing of the new water with existing tank contents that are most remote from the point of withdrawal. A preferred system would efficiently mix new water entering the tank with the existing tank contents thereby preventing stagnation. A preferred system would reduce the water age or residency time and related problems. A preferred system would eliminate the potential for ice cap formation.
The prior art recognizes the use of a plurality of inlet and outlet pipes, remote from each other in an attempt to promote mixing. Systems that have been proposed to date are typically ineffective or inefficient in that the water is not introduced properly and tends to short circuit or flow directly from the inlet to the outlet thus being unable to eliminate zones of stagnant water (“dead zones”) that occur in the reservoir. The prior art also recognizes attempts to improve the performance of the preceding by the addition of a directional elbow and a reducer on the inlet but this method, utilizing a reducer only does not provide a developed jet flow and further does not provide orientation, number and diameter of inlet pipes that are selected for best possible mixing for a specific tank geometry.
It is desirable to provide an inexpensive and easily maintained mixing system for use in reservoirs in order to reduce the potential for stagnation and excessive aging of the contained fluids and further to reduce the potential for the formation of dangerous ice caps.